Optimize crude preheat train to balance efficiency and operability

Critical attention must be paid when designing the crude
distillation unit (CDU) preheat train. Many pitfalls can easily
lead to unnecessary expenditures or unsatisfactory operations.
Key design criteria include high energy efficiency, operational
flexibility and reliability. Some designers may
elect to replicate a previously proven design, without full
consideration of the feedstocks characteristics. Others
may focus on implementing designs with computer-aided tools to
generate the heat exchanger network (HEN) without sufficient
attention to the effects of heat integration on the whole plant
operation. A practical and systematic design approach can
integrate process design, control, simulation and pinch
analysis, as outlined in Fig. 1.

Fig.
1. Crude preheat train design
flowchart.

Basis of (developed) design

A major energy demand within refineries is the required heat
input for the crude feedstock upstream of the CD column
to obtain the desired flash and distillation yields. Conversely,
heat removal is needed to provide the internal refluxes and
cooling of CDU products. The main design objective of the crude
preheat train is minimizing total energy consumption
by maximizing heat recovery. However, there is more to
designing a crude preheat train than the HEN alone.

When starting the design of the preheat train, the basic
parameters, such as feedstock, product yields and product
specifications are determined but other operational
philosophies must be developed for the engineering process.
Generally, the approach for a revamp design will be different
from a new unit. The revamp project must use many of the
existing constraints as set by the present configuration and
plant equipment.

Operational considerations. The refinerys operations team may
have strong views on the preheat train configuration,
especially if it is a revamp. Redundancy for some heat
exchanger services may be necessary to allow exchanger cleaning
to maximize CDU run length. Operators may also have strong
views on CDU controls and how the unit is started up and shut
down.

A strategy should be developed to start up the CDU and
vacuum distillation unit (VDU), and
operational needs should include:

Establishing hot and cold circulations to remove water
and to achieve on-specification products in both the CDU and
VDU.

Heating the feeds and cooling products. This may require
additional startup and shutdown lines and involve alternate
uses for equipment, especially for atmospheric and vacuum
residue.

Alternate routings may require more severe operating
conditions for some equipment and affect mechanical design
conditions.

If inter-unit heat integration will be considered, then the
shutdown schedule of all units will have to be compared and
reconciled. The design strategy should include operating
strategies so that processing operations can continue in the
event other process units are shut down. This may result in
additional equipment.

Basis (further development) of design

The overall philosophy for the heat integration must be
established. This could involve heat integration within the
unit or heat integration with other process units. Although
most consideration is given to intra-unit heat integration,
inter-unit heat integration is equally important for
total energy efficiency. Besides providing heat to the
liquefied petroleum gas (LPG) fractionation and gas-recovery
plants, heat integration with other refinery units should also
be considered. Excess heat from cokers and fluid catalytic
crackers (FCCs) is used for steam generation. These process
units are high-level heat sources and can be considered for
preheating the crude to reduce the overall refinery fuel
consumption. However, a careful analysis of the unit shutdown
philosophy should be evaluated. This could result in additional
investment for equipment to support refinery operation during unit
shutdowns and process upsets.

The refiner will generally have a payback philosophy; it can
be in terms of simple payback or internal rates of return on
incremental investment. For revamps, the extent of heat
recovery may be limited to a predetermined overall unit cost
budget. The owner may have energy targets or expectations for
the preheat train. This could be expressed in terms of total
energy targets for the unit, temperature approaches or target
heater inlet temperatures. Usually global (whole unit) targets
are set, with local minimum temperatures preset for individual
exchangers, which may be close to any temperature pinch.

Initial simulation analysis

For a new design, the target product cutpoints, feed/product
rates and specifications are set during the initial simulation
and generally are derived from a linear programming study
(Fig. 2). The total heating and cooling curves
are generated from the initial simulation. These curves are
used as a basis for a high-level heat integration
analysis.

Fig.
2. A typical CDU schematic.

At a high level, the total heating and cooling curves will
show if there is an excess or a shortfall in available heat
from the process to meet the preheating requirements of the
crude feedstock. Also, it can give an indication of the scope
for integration with other process units or facilities. The balance between heat
availability and demand depends on the crude feedstock. For
lighter crudes, more heat is recovered at lower temperatures,
and, consequently, lower preheat temperatures are usually
obtained, as shown in Fig. 3.

Fig.
3. Composite heating and cooling
curves.

It is likely that a stand-alone CDU will have a shortfall in
available heat from the hot streams within the unit. If this is
the case, energy (heat) input from other units to the CDU may
be feasible, and steam may be the best medium for reboiling the
light-ends columns in the gas plant, as there will be no
available heat from the CDU. However, where the CDU is
integrated with a VDU, excess heat may be available, which can
be used for reboiling light-ends columns, generating steam or
supplying heat to other users.

The effects from the number of pumparounds (PAs) and draw
temperatures on the heater inlet temperature should be
investigated to achieve an optimum design. The target global
approach temperature is set at the initial design stage.
Fig. 4 shows the results of this analysis.
This figure shows the optimum global temperature approach for a
particular network.

Fig.
4. Total cost targets for the CDU.

Startup and shutdown scenarios should be considered at an
early stage, especially when there is heat integration between
units. For example, if the CDU and VDU are heat-integrated,
then the VDU will receive hot feed directly from the CDU. While
this is good for heat integration, it does make starting up and
shutting down the unit more difficult, especially if the timing
of these operations is not identical.

For a revamp, modeling the present operation and
bench-marking the results against operating data are necessary
before simulating the new operations. Such evaluations are
helpful in rating the existing heat exchangers and determining
if the required heat inputs and removals can be achieved by the
present configuration. If a reconfiguration is required, a
review of the heating and cooling curves can provide an
overview of the heat-recovery possibilities and present
cross-pinch heat exchange. However, the constraints within the
existing configuration may dominate setting new targets.

Hydraulic analysis

The pressure profile must be developed and managed in
parallel with the heat integration. The preheat train will
often have multiple pumps and many heat exchangers in
seriesthus, the operating pressure may approach the
600-lb flange rating limits in some areas. Addressing the
pressure profile ensures that the design pressures remain below
key flange rating limits. This will give substantial capital
cost savings, compared with a design that exceeds the flange
rating limits. The practical design approach establishes the
pressure profile at an early stage; the pressure profile is
continually reviewed to ensure a good design.

The pressure profile starts at the crude-feed tank. The
first decision is whether the crude is blended and supplied
from a crude-blending tank or blended inline downstream of the
storage tanks. Inline blending is often accomplished by using
low-pressure (LP) blending pumps within the tank farm (some may
be on recycle), and a crude-charge pump boosts the pressure at
the CDU unit. The crude-charge pump allows the tank to be drawn
to a lower level, because the net positive suction head
(NPSH)1 required for the LP pumps is less than that
required for higher-pressure crude-charge pumps. The discharge
pressure of the blending pumps is more than adequate to meet
the NPSH requirements at the crude-charge pump suction. Using a
crude-charge pump at the CDU battery limit has the further
advantage that operating the unit on total recirculation
becomes straightforward and simplifies startup. Fig.
5 is a typical CDU preheat train; it has three main
sections:

Preheat before the desalter. The crude must
be heated to remove bottom sediment and water by the desalter.
Often, the temperature is allowed to float within an operating
range so that the crudes viscosity permits good water
removal, while ensuring the wet crude remains below its vapor
pressure. For a revamp, it may be necessary to control the
temperature more precisely to stay within operating
constraints. In some designs, booster pumps are installed
downstream of the desalters.

Flash drum/column. Following the desalter,
further heat recovery takes place. A flash drum column may be
installed, especially for lighter crudes in which a
high-preheat temperature is achieved causing high vapor
pressures.

Heater inlet. Finally, the crude is heated
up to the heater inlet temperature.

Fig.
5. A typical preheat train schematic.

In evaluating design pressures, heat exchanger burst-tube
scenarios may also need to be considered.

At the start of the design, a rough pressure profile should
be developed for the preheat train. It is based on an estimate
of the number of exchanger services in series. This will often
give a first indication that the configuration may need to be
changed to limit the pressure drop. The pressure profile should
be updated as the heat exchanger design progresses. For
revamps, the design pressure constraints can influence where
new exchangers can be fitted into the design. For a new unit,
the startup and shutdown requirements may influence the
decision on the location of any crude booster pumps.

It is usually preferable to set the pressure upstream of the
control valve at the inlet to any preflash/prefractionator; it
should be above the bubble-point pressure. This will suppress
vaporization, improve control of the operation, reduce the risk
of fouling by salt deposition, and avoid mechanical problems
due to slug flow. However, if the pressure balance is marginal
and two-phase flow is expected due to variations in the crude
feedstock, the unit must be designed
accordingly.

Two-phase issues

Operating within a two-phase flow regime can provide
advantages in terms of better heat recovery and lower operating
pressure. Units have operated successfully in this manner
without excessive fouling problems, provided that the upstream
desalting operation (double- or triple-stage desalting may be
required) is operated efficiently. If a unit is designed for
two-phase operation, then it is vital to consider the whole
range of crudes to be processed by the CDU. Operation with less
vaporization than the design basis will lead to lower fluid
velocities, poorer heat-transfer coefficients and lower
performance.

The operating pressure of the preflash/prefractionator will
affect the amount of light ends being flashed directly to the
crude column and the vaporization of crude from the heater, as
well as the CDU performance. The optimum pressure should be
selected based on a parametric analysis of the effect of
pressure on the crude column overflash/distillation and the efficiency of
the heat integration.

To ensure that the flowrate to the furnace is controllable,
the operating pressure should be high enough so that no
vaporization occurs upstream of the control valves on the
parallel furnace passes. For a new design, the operating
pressure should be set to provide a margin over the
bubble-point pressure and consider:

Clean and fouled heat exchanger operation

Potential use of heat exchanger bypasses

Preflash vessel operating pressure flexibility

Maximum Reid vapor pressure (Rvp) of the design
crude

Alternative crudes.

Also, low heater-flow trips only protect against low-pass
flow and not the total loss of crude flow. With the loss of
crude pressure, vaporization upstream of the flow orifices will
occur, and any flow trips will become unreliable due to the
higher vapor velocities.

Pinch analysis on the HEN design

Pinch analysis is a key design tool to achieve optimum
energy efficiency. Modern software allows a HEN to be generated
automatically for defined heating and cooling streams. However,
engineering judgment is critical to ensure a practical and
robust design. Designers should never rely solely on the
automatically generated solutions. The HEN generated by the
software may represent the best energy efficiency, but it can
often lead to a design with high capital requirements and
little flexibility. A practical design demands thorough
analysis, and it requires that the designers take charge to
configure the HEN, using the software only as a support tool.
In addition, commercially available software may have
limitations in the size/complexity of system it can
accommodate. For example, while a preheat train for a CDU may
be manageable, an integrated CDU and VDU may be too complex for
the software to analyze. Key points to consider for a practical
heat exchanger network design are:

The total heat recovery system design should be optimized
to achieve an economic target for energy usage when compared
to total capital expenditure (CAPEX) and should account for
both the preheat train and cooling equipment.

For inter-unit heat integration, there is a trade-off
between the total plant energy efficiency and operability.
HEN design software often presents solutions with a large
number of small heat exchangers and in an arrangement that
could be awkward to build and control. These designs should
be rationalized to obtain a good overall solution.

Where the achievement of the targeted cooling
temperatures is critical (for example to ensure a safe
rundown of products to tankage), cooling utilitiessteam
generation, cooling water or air coolingshould be used
for final cooling. Often, the trim coolers are sized, based
on the maximum rundown rates from different crude or cutpoint
operations or, for clean exchanger operations, when some
exchanger bypassing may be required to avoid temperature
pinch-out.

On larger-scale CDUs, it is common to split the feed
crude line into several parallel streams from section to
section. This ensures that heat exchangers remain within
their mechanical size limits, while increasing the level of
heat integration and enhancing heat recovery. However, an
increased number of splits can lead to operational and
control issues, especially during periods where the feed
crude ratio is being adjusted. There is always a trade-off
between energy efficiency and operability. However,
sophisticated control systems can be configured to use
ramping functions to help the operators optimize the splits
and smooth out the transitions during crude composition or
cutpoint changes.

The crude preheat upstream of the desalter needs careful
consideration to ensure that the temperature remains within
the desirable range120°C to 150°C. For revamps,
it may be necessary to control the desalter temperature more
tightly to avoid constraints. In this situation, one solution
is to split one of the duties into two, with one part duty
upstream of the desalter and the second part duty downstream.
This will allow adjustment of the desalter temperature
without significant effects on the heat recovery.

The HEN should be designed to allow the column
fractionation performance to be controlled by varying the
circulating reflux and/or quench duties. This is achieved by
varying the flowrate of the circulating refluxes and/or
providing a trim cooler in one or more PAs.

Temperature control by bypassing exchangers on the crude
side is not recommended due to increased fouling at low crude
velocities.

In the event of excess heat availability, recovery of
low-level heat from product rundown streams is preferable to
recovery from overhead streams, but this should be confirmed
by a review of the composite curves. Heat recovery from
overhead streams can lead to dewpoint corrosion problems and
potential contamination problems in the event of tube
leakage. If the exchangers are elevated, then significant
extra costs are incurred due to the need to mount the
exchangers in a structure. In some instances, running warm
side products to the receiving units will provide better
total heat integration. However, warm rundown
limits the heat that can be recovered from the rundown
streams, and heat recovery from overhead streams may need to
be considered.

For fouling services, if long run lengths are required,
then the exchangers should be placed in parallel/series
line-up to allow online isolation and cleaning. This is
particularly an issue with crude and residue streams.
Strategies include providing isolation block valves and
bypasses on both sides to allow for cleaning and designing
for increased flows through some exchangers while others are
out of service. Pressure drops will also increase with
fouling. While control valves will operate with a
considerable pressure drop at start of run, there will be
minimal pressure drop available at end of run.

As the HEN design progresses and the PAs are allocated
against crude splits, it is necessary to re-evaluate the PA
rates. Ideally for a particular heat exchanger service, the
mass rate, M, multiplied by the specific heat, Cp,
(M x Cp) for either side should be similar in value.
Thus, the heating and cooling curves will be approximately
parallel, and the heat exchanger area will be minimized. It is
possible to optimize the PA rates and crude splits, where there
are parallel exchangers, to minimize the heat exchanger area by
adjusting these rates.

Operational analysis

The single most important factor affecting the design of the
preheat train is the volume of each product. This is dependent
on the crudes characteristics. The optimized HEN can be
significantly different for light and heavy crudes due to the
differences in the heating and cooling curves. Some typical
crude yield variations are shown in Fig.
6.

Fig.
6. Some typical crude yield
variations.

As the crude-to-product-price differential dominates
refinery margins, refiners try to process less-expensive
(opportunity) crudes and/or to produce more higher-value
products to improve profitability. Therefore, the CDU feedstock may be expected to vary
significantly during the refinerys lifetime. Many
refiners try and cover this variation by specifying multiple
crudes and blends in the design basis. An alternate approach
would be to allow for higher vapor pressures, design
temperature and pressures, and higher heater duties in the
design to provide more operational flexibility.

If the unit is designed for more than one crude, then
different parts of the preheat train will be designed based on
different cases. The quickest option is to specify each
individual piece of equipment for the controlling case or
design mode. This typically results in the colder end of the
preheat train being designed for the lighter crudes and the
hotter end of the preheat train for the heavier crudes. The
unit controls should be capable of compensating for the
resulting over-design, and the designer will gain an
understanding of the performance by converting the simulation
into rating mode. In rating mode, the key equipment parameters
such as heat exchanger geometries or, more simply, heat
transfer coefficient, U, times area, A,(U x
A)values are input into the simulation, and the
uncontrolled process conditionsincluding temperatures,
pressures and heat exchanger dutiesare resultant from the
specified equipment data.

Rating mode can be used to simulate the unit operation
(including over-design) for the normal operating cases, with
the exchangers in a clean, or a moderately or heavily fouled
state, or at turndown. Such simulations would allow the
controls to be tested for a range of scenarios, and may
identify that over-performance in one area results in poorer
performance in another, due to temperature pinch-out. This may
result in a need for additional bypasses or an increase in some
utility cooling duties. In some cases, the utility cooling
design duties may be set by the rating cases.

The other major use of rating simulations is to reduce the
net over-design resulting from the overlay of several design
cases. This is done by trial and error, based on assessing the
effects of the over-design from one case on the other cases. An
ideal scenario would be for the design area for each case to be
approximately the same, and with no additional area having to
be added to the utility cooling design cases. This last option
can be time-consuming and costly at the design stage, but could
give significant CAPEX benefits. HP

1 The NPSH is the difference in meters (or feet)
of fluid between the absolute pressure at the pump suction,
taking into account the tank level minus the pressure drop in
the suction pipe and the vapor pressure of the fluid.

The authorJenny Zhang is a principal process
engineer working as a member of the Process Technology Group within
Foster Wheelers Process Engineering Department
in Reading, UK. Ms Zhang has over 20 years of
experience, specializing in process design,
simulation, process RAM studies and energy
optimization covering refining, LNG, GTL and CCS
projects. She holds an
MPhil degree in chemical engineering from UMIST,
Manchester, UK, and a BSc degree in chemical
engineering from East China University of Science and
Technology in Shanghai,
China.

Have your say

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Excellent article. It may be necessary to calculate the payout to include gross overhead in the preheat train when rundown products and PA's mai not be enough to reached a prehet train outlet temperature. Do you know who is the licensor of HEN software and b) there are any other software to calculate the preheat train?

DANIEL VARGAS08.21.2013

The article summarizes properly the main variables that process design engineers should take into account during design of a new preheating train for CDU. it should be done integration both, design and operational experience. We execute something similar in a new design, evaluating the heat integration between CDU / VDU and deep conversion unit. As the author recommends, we used the software only as a tool, while the HEN was modified manually based on a trade-off between operational philosophies and energy efficiency. Further evaluations were done based on startup, turndown, shutdown and emergency cases, then, the final HEN, with the additional connections and over design for some specific services, used for alternative cases, were proposed.

ROBERTO ABARILLA06.29.2013

Great article. A simple system (Crude preheating) yet complex with many factors to consider. Honestly, I have not consider the effects of start-up and shutdown activities in my previous design. Many thanks.